This invention relates to cast composite materials, and, more particularly, to the preparation of such composite materials having matrix alloys that do not readily wet the reinforcement particles.
Cast composite materials are conventionally formed by melting a matrix alloy in a reactor and then adding particles. The mixture is vigorously mixed to encourage wetting of the matrix alloy to the particles, and after a suitable mixing time the mixture is cast into molds or forms. The mixing is conducted while minimizing the introduction of gas into the mixture. The cast composite materials have fully wetted particles, few voids, and a generally uniformly mixed structure. Complete wetting is necessary to realize the full composite strength and other mechanical properties.
Such cast composite materials are much less expensive to prepare than other types of metal-matrix composite materials such as those produced by powder metallurgical technology. Composite materials produced by this approach, as described in U.S. Pat. Nos. 4,759,995 and 4,786,467, have enjoyed commercial success in only a few years after their first introduction.
As the cast composite materials have entered commercial production, customers have sometimes requested particle/matrix alloy combinations wherein the matrix does not readily wet the particles. In other instances, new metallic alloys have been identified that produce unexpectedly superior performance when used as the matrix phase of the composite materials, except for the problem that the composite materials are difficult to produce commercially due to the inability of the matrix alloy to wet and mix with the particles readily.
There are a number of techniques that can be applied to enhance wetting, which may work in some circumstances. The particles can be modified with special coatings, but the coating operation can significantly raise the cost of the particles and the composite material. Small amounts of reactive gases can be introduced into the mixing chamber, but the improved wetting may only be achieved at the cost of increased porosity in the cast composite material. Special reactive alloying ingredients can be added to the melt, but these are often expensive and may have adverse consequences in the production of undesired minor phases in the cast composite material. Another approach is to raise the temperature at which the mixing to achieve wetting is accomplished, but increased temperature may also result in the acceleration of the production of deleterious minor phases where such phases are thermodynamically favored but kinetically slow in forming at lower temperatures.
There therefore exists a continuing need for an improved technique for producing cast composite materials from par title/matrix alloy combinations wherein the matrix does not inherently readily wet the particle. Desirably, any such technique would not add substantially to the cost of the product or have detrimental effects The present invention fulfills this need, and further provides related advantages.